1. Field of the Invention
The present invention relates to medium or high power a.c. switches and more particularly to the realization in monolithic form of an a.c. switch triggerable only during a predetermined half-period and remaining automatically conductive during the next half-period.
2. Discussion of the Related Art
Presently, the triac, whose conduction state is triggered by a signal applied between its gate and one of its main terminals, is a commonly used a.c. switch. This switch is blocked again at each zero crossing of the current; hence, it has to be controlled at each half-period.
The operation of a triac is schematically illustrated with reference to FIGS. 1A-1D where FIG. 1A shows the mains voltage waveform, FIG. 1B shows the control pulses I.sub.g provided to the triac gate, FIG. 1C shows the current flowing through the triac, and FIG. 1D shows the voltage across the triac. For the sake of simplification, it is assumed in this example that there is no phase shift between the voltage and the current.
It can be appreciated that, if a control pulse I.sub.g is applied during a positive or negative half-period of the mains voltage, the triac goes into conduction mode and that it is blocked again at each zero crossing of the current and remains blocked if no control pulse is applied thereto.
Such a component has proven efficient for power regulating in phase control mode. Some drawbacks, however, are inherent in the triac structure and in some applications.
The geometrical structure of a triac is complex, particularly regarding the areas corresponding to the gate region, which renders manufacturing of the triac difficult.
This complex geometrical structure limits the performances of the triac which generally has a poor gate sensitivity. So, in some applications where a high sensitivity is required, a thyristor combined with a rectifying bridge is substituted for the triac, that is, a monolithic component is not used.
In some applications, for example when the switch is disposed in series with a transformer, it is more advantageous, after a quiescent phase, to trigger the triac again during a half-period having a polarity opposite to the polarity of the last conduction phase in order to avoid the drawbacks associated with an excessive current surge caused by magnetization/demagnetization phenomena. Because of these phenomena, it is compulsory to combine the triac with complex control circuits which store the polarity of the last conduction half-period and authorize a next triggering only during a half-period having the opposite polarity.